Removing the antenna from the base:
I found it easier to stand at the rear of the car. Once unscrewed so the antenna part just rotates on the base and wont unscrew any more (anti chav-theft feature), twist it down towards the back of the car (so its now horizontal) and give it a good tug, when the antenna will separate from the base.
A handful of the stubby type aerials on EBay fit just fine and line up nicely with the base.
Removing the antenna Base from the roof! :
...With the tailgate opened, pull the rear of the headlining out of the rubber seal, around the bootlid area.
It might be lightly bonded to the roof too, so loosen a bit with your hand, as you work inwards, until you get to the aerial base Bolt !
There are 2 wires coming off the Base, which are secured by 2 White clips, right up at the back of the roof.
Undo the Bolt, and pull the Base away from the roof from the top, threading the 2 leads/connectors through the hole in the roof as well.